1. Field of the Invention
This invention relates to methods of manufacture of semiconductor memory devices and the devices manufactured thereby and more particularly to flash memory devices and methods of manufacture thereof.
2. Description of Related Art
U.S. Pat. No. 5,612,249 of Sun et al. for "Post-Gate LOCOS" shows a process comprising: etching a gate oxide layer and a polysilicon layer to form gate structures for the FETs; surrounding the gate structures with an oxidation mask; etching areas of the top surface of the substrate which do not include the gate structures of the FETs such that the substrate is thicker underneath the gate structures for the FETs than it is at other areas of the substrate; and forming the LOCOS field oxide isolation regions by growing an oxide layer between the FETs. Sun patent '249 shows the concept of using isolation to self-align the S/D regions; but has the disadvantage that a complicated process is described requiring a series of steps to pattern the gates and form the trenches.
Aritome et al. for "A 0.67 .mu.m.sup.2 Self-Aligned Shallow Trench Isolation Cell (SA-STI Cell) for 3 V-only 256 Mbit NAND EEPROMS" IEDM 94-61 Pages 3.6.1-3.6.4 describes a floating gate that is "self-aligned with the STI."
U.S. Pat. No. 5,120,671 of Tang for a "Process for Self Aligning a Source with a Field Oxide Region and a Polysilicon Gate" and U.S. Pat. No. 5,376,573 of Richart et al. for a "Method of Making a Flash EPROM Device Utilizing a Single Masking Step for Etching and Implanting Source Regions within the EPROM Core and Redundancy Areas" shows a method for forming contact trenches and isolation trenches. The above Tang and Richart et al. references are based on LOCOS for isolation.
U.S. Pat. No. 5,217,920 of Mattox et al. shows a "Method of Forming Substrate Contact Trenches and Isolation Trenches Utilizing Anodization for Isolation" discloses features which differ from the present invention in that it provides for a buried contact.
FIGS. 1A-1G show processing steps for a prior art method for manufacturing a self-aligned floating gate flash memory cell. Some problems with the process are as follows:
1. Two separate photolithography masks are required to define active area and floating gate electrode, respectively. PA1 2. Misalignment between these two photolithography masks limits the shrink scale for a Flash memory cell. PA1 3. The LOCOS method limits the shrinkage of the width of the active area for Flash memory cells. PA1 1. Only a single masking layer is required instead of two conventional layers to define an active area and a floating gate electrode. PA1 2. The problems associated with a misalignment between the two masks usually employed for defining the active area and the floating gate electrode is completely avoided and the space between the active areas of two adjacent cell can be scaled down, easily. PA1 3. Because of the planarization of device topography, the following etch steps, for example: stacked gate electrode etch and contact hole etch, become much easier. PA1 4. Application of trenched source line implant can save the space between stacked gate electrode.
The process with the device 8 as seen in FIG. 1A begins with silicon substrate 10 with a pad silicon oxide layer 11 formed thereon. Above pad oxide layer 11 are formed silicon nitride masking strips 12A, 12B, which are employed during formation of the active region of the device employing the well known LOCOS (LOCal Oxidation of Silicon) process.
In FIG. 1B, the device of FIG. 1A is shown after a silicon nitride masking strips 12A and 12B are then stripped from the device 8 leaving a structure comprising a series of parallel FOX (Field OXide) region strips 16A, 16B and 16C formed on substrate 10.
As shown in FIG. 1C a tunnel oxide layer 17 has been formed on the exposed surface of the substrate 10 of FIG. 1B, aside from the FOX region strips 16A, 16B and 16C. A blanket doped polysilicon layer 18 is formed on device 8.
FIG. 1D shows the device of FIG. 1C after a floating gate mask has been formed to pattern the layer 18 into floating gate electrodes 18A and 18B bridging between FOX region strips 16A and 16B on the left and between and FOX region strips 16B and 16C on the right. There is the potential of misalignment with the two masking steps required to achieve the results shown in FIGS. 1B and 1D.
In FIG. 1E the device of FIG. 1D is shown after an interpolysilicon dielectric, ONO, layer 20 is grown. Then as shown a second polysilicon layer 22 is formed over the ONO layer 120 and a tungsten silicide layer 24 formed over the second polysilicon layer 22. Layer 22 and layer 24 are to be patterned into control gate electrodes stacks 22/24 on a step-by-step basis.
FIG. 1F shows the device of FIG. 1E after a Stacked Gate Etch (SGE) mask (not shown) has been used to etch away exposed portions of layers on the periphery of device 8 after the silicide layer 24, polysilicon 22, ONO layer 20 and first polysilicon layer 18A, 18B through that mask yielding the structure seen in FIG. 1F with the surface of the FOX regions 16A, 16B and 16C exposed beyond the remaining gate electrode stack 28.
FIG. 1G shows the device of FIG. 1F after the self-aligning source mask has been used to etch away the exposed portions of the FOX regions 16A, 16B and 16C to expose the surfaces of the substrate to form S (source) regions 30A and 30B adjacent to the floating gates 18A and 18B. The drain regions (not seen) are formed on the opposite side of the gate electrode stack 28 from the source regions 30A and 30B.